Home > Articles > Web Development > Perl

  • Print
  • + Share This
This chapter is from the book

3.4 Testing Legacy Code

"This is all well and good," I can hear you say, "but I just inherited a swamp of 27 programs and 14 modules and they have no tests. What do I do?"

By now you've learned that it is far more appealing to write tests as you write the code they test, so if you can possibly rewrite this application, do so. But if you're stuck with having to tweak an existing application, then adopt a top-down approach. Start by testing that the application meets its requirements . . . assuming you were given requirements or can figure out what they were. See what a successful run of the program outputs and how it may have changed its environment, then write tests that look for those effects.

3.4.1 A Simple Example

You have an inventory control program for an aquarium, and it produces output files called cetaceans.txt, crustaceans.txt, molluscs.txt, pinnipeds.txt, and so on. Capture the output files from a successful run and put them in a subdirectory called success. Then run this test:

Example 3.6. Demonstration of Testing Program Output

1  my @Success_files;
   2  BEGIN {
   3    @Success_files = glob "success/*.txt";
   4  }
   6  use Test::More tests => 1 + 2 * @Success_files;
   8  is(system("aquarium"), 0, "Program succeeded");
   10 for my $success (@Success_files)
   11 {
   12   (my $output = $success) =~ s#.*/##;
   14   ok(-e $output, "$output present");
   16   is(system("cmp $output $success > /dev/null 2>&1"),
   17      0, "$output is valid");
   18 }

First, we capture the names of the output files in the success subdirectory. We do that in a BEGIN block so that the number of names is available in line 6. In line 8 we run the program and check that it has a successful return code. Then for each of the required output files, in line 14 we test that it is present, and in line 16 we use the UNIX cmp utility to check that it matches the saved version. If you don't have a cmp program, you can write a Perl subroutine to perform the same test: Just read each file and compare chunks of input until finding a mismatch or hitting the ends of file.

3.4.2 Testing Web Applications

A Common Gateway Interface (CGI) program that hasn't been developed with a view toward automated testing may be a solid block of congealed code with pieces of web interface functionality sprinkled throughout it like raisins in a fruit cake. But you don't need to rip it apart to write a test for it; you can verify that it meets its requirements with an end-to-end test. All you need is a program that pretends to be a user at a web browser and checks that the response to input is correct. It doesn't matter how the CGI program is written because all the testing takes place on a different machine from the one the CGI program is stored on.

The WWW::Mechanize module by Andy Lester comes to your rescue here. It allows you to automate web site interaction by pretending to be a web browser, a function ably pulled off by Gisle Aas' LWP::UserAgent module. WWW::Mechanize goes several steps farther, however (in fact, it is a subclass of LWP::UserAgent), enabling cookie handling by default and providing methods for following hyperlinks and submitting forms easily, including transparent handling of hidden fields.9

Suppose we have an application that provides a login screen. For the usual obscure reasons, the login form, login.html, contains one or more hidden fields in addition to the user-visible input fields, like this:

 <INPUT NAME="username" TYPE="text">
 <INPUT NAME="password" TYPE="text">
 <INPUT NAME="fruglido" TYPE="hidden" VALUE="grilku">
 <INPUT TYPE="Submit">

On successful login, the response page greets the user with "Welcome, " followed by the user's first name. We can write this test for this login function:

Example 3.7. Using WWW::Mechanize to Test a Web Application

1  #!/usr/bin/perl
   2  use strict;
   3  use warnings;
   5  use WWW::Mechanize;
   6  use Test::More tests => 3;
   8  my $URL = 'http://localhost/login.html';
   9  my $USERNAME = 'peter';
   10 my $PASSWORD = 'secret';
   12 my $ua = WWW::Mechanize->new;
   13 ok($ua->get($URL)->is_success, "Got first page")
   14   or die $ua->res->message;
   16 $ua->set_fields(username => $USERNAME,
   17                 password => $PASSWORD);
   18 ok($ua->submit->is_success, "Submitted form")
   19   or die $ua->res->message;
   21 like($ua->content, qr/Welcome, Peter/, "Logged in okay");

In line 12 we create a new WWW::Mechanize user agent to act as a pretend browser, and in line 13 we test to see if it was able to get the login page; the get() method returns a HTTP::Response object that has an is_success() method. If something went wrong with fetching the page the false value will be passed through the ok() function; there's no point in going further so we might as well die() (line 14). We can get at the HTTP::Response object again via the res() method of the user agent to call its message() method, which returns the text of the reason for failure.

In lines 16 and 17 we provide the form inputs by name, and in line 18 the submit() method of the user agent submits the form and reads the response, again returning an HTTP::Response object allowing us to verify success as before. Once we have a response page we check to see whether it looks like what we wanted.

Note that WWW::Mechanize can be used to test interaction with any web application, regardless of where that application is running or what it is written in.

3.4.3 What Next?

The kind of end-to-end testing we have been doing is useful and necessary; it is also a lot easier than the next step. To construct comprehensive tests for a large package, we must include unit tests; that means testing each function and method. However, unless we have descriptions of what each subroutine does, we won't know how to test them without investigative work to find out what they are supposed to do. I'll go into those kinds of techniques later.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020